The field of this invention is the detection of a constituent in a fluid or of a condition in an atmosphere.
Undesirable and often dangerous gases such as carbon monoxide or hydrogen sulfide are often by-products of industrial plants such as oil refineries or chemical plants. These constituents should be detected quickly and accurately in order to prevent harm to workmen in the immediate area. Also, it is sometimes desirable to inject particular quantities of a constituent into a fluid, and the exact concentration must be accurately measured.
U.S. Pat. No. 3,464,799, invented by the inventor herein discloses a device for measuring the actual concentration of an undesirable constituent such as hydrogen sulfide in a gaseous medium. This device principally utilizes a chemical reaction type sensor--a paper strip impregnated with proper chemicals to provide discoloration upon reaction with a particular constituent in the gaseous medium. The paper strip is mounted on a reel in a substantially sealed housing such that only a particular portion of the tape is exposed to the atmosphere at any time. The discoloration on the tape is measured by an electrical circuit utilizing a light source directed against the area on the paper strip where the hydrogen sulfide has reacted; and, a measurement circuit reads the magnitude of the light reflected off of the paper strip and thus the magnitude of the concentration of the hydrogen sulfide on the strip.
Although the device of U.S. Pat. No. 3,464,799 has been found to be very reliable, it has been found necessary to improve upon this device in order to eliminate some problems inherent with attempting to measure the concentration of a particular constituent. For example, it is generally necessary to provide a continuing reference base in order to properly read concentration. Secondly an inherent limitation in measuring concentration has been that a considerable time has been required for equilibrium to be reached and a measurement obtained. Also, reversible reactions have been a problem where the chemical reaction of the constituent on the sensing material begins to reverse and thus fails to indicate the actual magnitude of the concentration of the constituent in the atmosphere. Another difficulty is the inherent background noise found in measuring an absolute such as magnitude of the reaction of the constituent on the sensing material.